The invention relates to a friction element having a base element on which a coating is applied by thermal spraying of a copper alloy.
The invention further concerns a method for producing a friction element in which a coating is applied onto a base element by thermal spraying of a copper alloy.
A friction element of this kind, and a method of this kind for producing a friction element, are known from DE-C 3 637 386.
According to these, for the production of a synchronizer ring which consists substantially of a toroidal metal body having an adjoined external tooth set, a base adhesion layer made of a bronze alloy, an aluminum/nickel alloy, or the like is first applied onto the metal base element using the plasma spray method.
A scatter-sintered friction lining is then applied onto this base adhesion layer. For this, the base element is assembled to an insert element. The insert element has centering means for retaining the base element in a defined position, and a countersurface arranged at a defined spacing from the surface. The cavity constituted by the defined spacing, the surface, and the countersurface is filled with a scatter-sintered powder, and the insert element is sintered to the base element when the cavity is filled.
In this fashion, a friction lining produced by the sintering method can be attached to the base element without additional mechanical immobilization means.
Many other methods are also known for producing scatter-sintered friction linings (cf., for example, DE-C 3 417 813 and EP-B-0 292 468).
An additional friction layer of molybdenum must usually also be applied onto the scatter-sintered friction linings in order to achieve the necessary service life (cf. DE-A-2 055 346, DE-C-3 412 779).
The production of scatter-sintered friction linings is thus relatively complex and expensive.
It is furthermore fundamentally known to use, as the material for single-metal synchronizer rings, special brasses of the CuZn40A12 type, since these materials are distinguished by a good combination of coefficient of friction, wear resistance, and mechanical strength. For synchronizer rings subject to greater mechanical stresses, special brasses containing manganese, aluminum, iron, silicon, nickel, tin, and/or lead are used (DE-C 3 412 779). Here again, the necessary wear resistance of these synchronizer rings that can carry higher mechanical loads is achieved by means of an applied molybdenum coating.
A feature common to the previously known synchronizer rings is the fact that a series of complex process steps is necessary in order to produce synchronizer rings for high loads, generally including the application of a molybdenum coating since only a molybdenum coating has the necessary wear resistance required, among other applications, for synchronizer rings that are to be used in a motor vehicle transmission. In addition, a complex finish treatment by means of grinding is generally necessary in order to achieve the necessary surface quality for the friction surface. Coating with molybdenum has, however, recently proven problematic for various reasons. On the one hand, molybdenum is a comparatively expensive metal which requires complex and expensive tools for machining because of its high wear resistance. On the other hand, the result of using molybdenum as a wear-resistant layer on synchronizer rings is that the transmission in question must be treated as special waste when later disposed of. An effort has therefore recently been made to find ways of developing synchronizer rings in which a wear-resistant molybdenum coating can be dispensed with.